1. Field of the Invention
This invention involves the growth of native films.
2. Description of the Prior Art
The rapid increase in large scale and very large scale integrated devices has been made possible in large part as a result of improved thin film growth and processing technology. Such films can now be grown and processed on semiconductor substrates with very high resolution. Needless to say, such growth and processing steps must take place without the formation of impurities which would drastically interfere with the electrical properties of the resulting devices. Such undesirable elements may appear in the form of ambient atoms and molecules, or may result from imperfect or incomplete chemistry which may occur during the growth or processing steps.
An initial step in the fabrication of integrated semiconductor devices usually involves the growth of thin films upon a semiconductor substrate. Such films may either be deposited or may be grown from the substrate. The growth of exemplary oxide films, commonly referred to as native oxide films, occurs when the semiconductor substrate is exposed to oxygen bearing compounds which, under the proper electrical, thermal and/or chemical conditions, diffuse through the semiconductor and react with it to form an oxide.
When a film is grown on a compound semiconductor, any deviation from exact stoichiometry or any unreacted constituent (both referred to here generally as "astoichiometry") in the film, or in the interface region between the film and the underlying semiconductor, can have deleterious effects on the electrical performance of the resulting device. Undesirable astoichiometry usually can be traced to the complicated chemistry involved in the growth of the compound semiconductor film. Specifically, each of the various constituents of the compound substrate will most often have different reaction properties for the chemistry utilized during the growth. Consequently, the constituents of the compound semiconductor will not react uniformally and the reacted material will be astoichiometric, e.g., interspersed with unreacted semiconductor constituents. Relevant reaction parameters which might vary, depending on the constituents of the compound semiconductor, include chemical reaction rates, diffusion rates and volitization parameters.
Astoichiometric effects in the growth of native compound semiconductor films may reveal themselves in both the film, and the interface region between the film and the substrate. The effects of such astoichiometry may be exemplified by the interface region which forms when a native gallium arsenide oxide film is grown using the specific plasma oxidation technique described in U.S. Pat. No. 4,062,747. This growth process involves exposing the gallium arsenide oxide substrate to an appropriate plasma, containing oxygen-bearing compounds. It is found that devices grown in this manner may contain excess unreacted arsenic in both the interface region and the film unless specific preventive steps are taken. Such steps are disclosed in U.S. Pat. No. 4,062,747 and in U.S. application Ser. No. 810,771 filed June 28, 1977. Unless the excess arsenic is minimized, large interface state densities result and significant hysteresis appears in the associated C-V curves. The deleterious arsenic is stable even under high N.sub.2 temperature anneal. In fact, upon annealing in nitrogen gas, the arsenic interface layer crystalizes into a metallic domain at the interface, thus severely altering the electrical communication between the oxide and semiconductor.
As mentioned above, prior art techniques have been suggested for minimizing any imbalance which might tend to appear in the interface region. Among such preventive steps are annealing in hydrogen at 450 degrees C. Under these conditions, the hydrogen apparently reacts with the oxide and the arsenic layer to essentially eliminate the hysteresis in the C-V curve. However, such annealing also causes the oxide to become leaky, probably because the hydrogen-bearing compounds in the oxide are not good dielectrics. Alternatively, film parameters can be altered by rapid growth (i.e., .about.500 A/min), and surface films which act as filters can be advantageously utilized to reduce the arsenic accumulation at the interface. However, these methods have not been successful in simultaneously eliminating the hysteresis in the C-V curves and preventing the oxide from being leaky.